In the offshore petroleum industry it is well known to use tensioner systems on floating drilling rigs and other vessels, in order to maintain a pre-selected vertical tension in a marine riser extending from the rig and down to a subsea wellhead. When the vessel is heaving and rolling due to waves, currents and winds, the tensioner system will try to keep constant tension in the riser.
One type of tensioner system which is known in the art, is termed a “direct acting tensioner” (DAT) system. In a typical arrangement on a drilling vessel, a DAT system basically comprises a number of hydraulic-pneumatic cylinders suspended underneath the drill floor in a circle-symmetrical configuration above the lower deck. The cylinders' free (lower) ends are connected to a so-called tensioner ring, which may be connected to a telescopic joint which in turn is connected to the marine riser.
When the DAT system is not in use and not connected to the telescopic joint, the tensioner assembly is “parked” in a location away from the well centre, on the x-mas tree side or on the BOP side, where it does not interfere with other operation taking place above or through the moonpool. However, as the drilling rig may be moving considerably in waves and swell, the cylinders (and thus the tensioner ring) of a parked DAT system is susceptible of swinging uncontrolled back and forth, with the risk of damaging adjacent equipment—as well as the cylinders themselves—and causing harm to personnel.
Methods and means of DAT system seafastening exist, commonly employing an arrangement of wires and winches. The known systems are, however, cumbersome and time consuming to connect and activate. In addition, the prior art seafastening systems induce large, undesired, forces on the cylinders and/or packing boxes.
The uncontrolled movement of the DAT system also makes connecting the tensioner ring to the telescopic joint difficult and potentially dangerous.
Another problem with DAT systems arises when the tensioner ring is connected to the telescopic joint and the DAT system is in operation: Due to the rig motions, the hydraulic and/or pneumatic hoses extending from the control systems and to each of the tensioner cylinders are swinging about in the moonpool in an uncontrolled manner, and are often damaged.
The state of the art includes US 2010/0047024 A1 (Curtiss) which describes an apparatus to restrain a riser tensioner of an offshore drilling rig and which includes a restraint cone configured to fit within hydraulic cylinders of the riser tensioner, a hoist configured to extend and retract the restraint cone, and a tension member extending from a lower end of the restraint cone, the tension member configured to engage a lower end of the riser tensioner and maintain a wedging action between the restraint cone and the hydraulic cylinders.
The present inventor has devised and embodied this invention to overcome these shortcomings and to obtain further advantages.